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DNA Methylation 1 Epigenetics • Epigenetics: the passing on of modified inheritable traits to subsequent generations without changing the DNA sequence per se... Epigenetic trait modification can be triggered by environmental factor(s). • Pembrey (a geneticist at University College London in the U.K.) and his colleagues (who studied the unusually detailed historical medical records of the isolated northern Swedish city of Overkalix) found that grandsons of men who experienced famine during mid-childhood went through puberty earlier and had longer lifespans, while the grandsons of men who were well fed in early childhood had an increased likelihood of diabetes. • For females, the effect was similar but it was tied to the grandmother, rather than the grandfather. • More contemporaneously, Pembrey has found that fathers who had started smoking before age 11 had sons who were significantly fatter than average. There was no similar effect on daughters Epigenetics • Epigenetics: the passing on of modified inheritable traits to subsequent generations without changing the DNA sequence per se... Epigenetic trait modification can be triggered by environmental factor(s). • Pembrey (a geneticist at University College London in the U.K.) and his colleagues (who studied the unusually detailed historical medical records of the isolated northern Swedish city of Overkalix) found that grandsons of men who experienced famine during mid-childhood went through puberty earlier and had longer lifespans, while the grandsons of men who were well fed in early childhood had an increased likelihood of diabetes. • For females, the effect was similar but it was tied to the grandmother, rather than the grandfather. • More contemporaneously, Pembrey has found that fathers who had started smoking before age 11 had sons who were significantly fatter than average. There was no similar effect on daughters Figure 1. Epigenetic inheritance via the female and male germ line. If a gestating mother (F0, blue) is exposed to environmental stressors, her fetus (green) and its already developing germ line (red) are also directly exposed. As a result, phenotypes observed up to the F2 generation (red) may result from the grandmother’s experience. The F3 generation (yellow) is the first generation that has not been exposed to these environmental stressors, thus phenotypes observed in this generation could represent transgenerational epigenetic inheritance. If inheritance via the male or the maternal germ line before gestation is investigated, environmental stressors may affect the F0 generation (blue) and the developing germ line (red). As a result, the first generation that could represent transgenerational epigenetic inheritance is F2 (yellow). • Angelman syndrome results from a loss of gene activity in a specific part of chromosome 15 (the 15q11-q13 region). – This region contains a gene called UBE3A that, when mutated or absent, likely causes the characteristic features of this condition. People normally have two copies of the UBE3A gene, one from each parent. – Both copies of this gene are active in many of the body's tissues. – In the brain, however, only the the maternal copy is active. If the maternal copy is lost because of a chromosomal change or a gene mutation, a person will have no working copies of the UBE3A gene in the brain. • Prader-Willi syndrome is caused by the loss of active genes in a specific part of chromosome 15, the 15q11-q13 region. People normally have two copies of this chromosome in each cell, one copy from each parent. – Prader-Willi syndrome occurs when the paternal copy is partly or entirely missing 6 Epigenetics • Epigenetics: the passing on of modified gene functions to subsequent generations without changing the underlying DNA sequence. ... Epigenetic modifications can be triggered by environmental factors. • Pembrey (a geneticist at University College London in the U.K.) and his colleagues (who studied the unusually detailed historical medical records of the isolated northern Swedish city of Overkalix) found that grandsons of men who experienced famine during mid-childhood went through puberty earlier and had longer lifespans, while the grandsons of men who were well fed in early childhood had an increased likelihood of diabetes. • For females, the effect was similar but it was tied to the grandmother, rather than the grandfather. • More contemporaneously, Pembrey has found that fathers who had started smoking before age 11 had sons who were significantly more obese than average. There was no similar effect on daughters Epigenetic Effects Can Be Inherited • Epigenetic effects can result from modification of a nucleic acid after it has been synthesized or by the perpetuation of protein structures. • Epigenetic effects may be inherited through generations (transgenerational epigenetics). • The two pictures show the eyes of two genetically identical flies. The difference in eye colour is determined by epigenetic factors. (Credit: Renato Paro/ETH Zürich) 9 Epigenetic affects of maternal diet... ScienceDaily (Mar. 9, 2011) — Poor diet during pregnancy increases offspring's vulnerability to the effects of aging, new research has shown for the first time. To test their theory, the researchers used a well-established rat model where, by altering the protein content of the mother's diet during pregnancy, the offspring develop type 2 diabetes in old age. First, they studied the RNA from insulin secreting cells in the pancreas from offspring of normally fed as well as malnourished mothers in young adult life and in old age. When they compared the two, they found that there was a significant decrease in the expression of the Hnf4a gene in the offspring prone to type 2 diabetes. The expression of Hnf4a also decreased with age in both groups. Second, they studied the DNA and found that the decrease of Hnf4a was NOT caused by any genetic changes, but rather by some epigenetic changes...... Caring Mothers Reduce Response to Stress for Life How a rat responds to stress depends on whether its mother cared for it properly as a pup, which marks its genes for life. Dr. Mae-Wan Ho reports Maternal effects in the spotlight Maternal effects on the development of offspring are well known. But they are thought to be due to nutritional and physiological factors affecting the foetus in the womb; and within the past few years, geneticists have discovered that diet and stress can profoundly change the pattern of gene expression in the offspring, affecting their health prospects as adults (see Diet trumping genes, SiS 20). A team of researchers from the Douglas Hospital Research Centre and McGill University in Montreal Canada, and the Molecular Medicine Centre, in Edinburgh University Western General Hospital in the UK, now report a remarkable experiment in which the behaviour of the mother nursing her pups not only affects the pups’ response to stress as adults, but are correlate with changes in gene expression states in brain cells that persist into adult life. Such changes are referred to as ‘epigenetic’ as they do not involve alterations in the base sequence of DNA in the genome, only their off and on states; but they can persist in the brain cells and are passed on to all the daughter cells. Caring mothers reduce stress response of pups In the nest, the mother rat licks and grooms her pups and, while nursing, arches her back to groom and lick her pups. Some mothers (high performers) tend to do these acts more frequently than others (low performers). As adults, the offspring of high performers are less fearful and show more modest responses to stress in the hypothalamus-pituitary-adrenal (HPA) neuro-endocrine pathway. ISIS Report113 07/09/04 Four Recognized Mechanisms of Cellular Epigene<c Inheritance Systems • Self sustaining feedback loops • 3 D templa<ng -Structural Inheritance • Chroma<n Marking • RNA-mediated EIS 12 Four Recognized Mechanisms of Cellular Epigene<c Inheritance Systems • Self sustaining feedback loops • 3 D templa<ng -Structural Inheritance • Chroma<n Marking • RNA-mediated EIS 13 Four Recognized Mechanisms of “Cellular Epigene<c Inheritance Systems” • Self sustaining feedback loops -protein products (or modifica<ons thereof) or mRNA concentra<ons… – Candida albicans, where an epigene<c switch underlies the transi<on between white and opaque cells—two states that are heritable for many genera<ons due to the presence of a protein (Wor1) that gets diluted over genera<ons… leading to a change of cell type back to “white”. • 3 D templa<ng -Structural Inheritance • Chroma<n Marking • RNA-mediated EIS 14 15 Four Recognized Mechanisms of Cellular Epigene<c Inheritance Systems • Self sustaining feedback loops • 3 D templa<ng -Structural Inheritance – Inheritance of alterna<ve three-dimensional (3-D) structures through spa<al templa<ng: Prions • Chroma<n Marking • RNA-mediated EIS 16 • prion – A proteinaceous infectious agent that behaves as an inheritable trait, although it contains no nucleic acid. – Examples are: – Bovine spongiform encephalopathy (BSE), commonly known as mad cow disease, and Scrapie in sheep a both fatal, neurodegenerative diseases that cause a spongy degeneration in parts of the brain and spinal cord -caused by an abherant form of a protein PrPSc. – Sup35 in Yeast – PrPSc, the agent of scrapie in sheep and bovine spongiform encephalopathy in cattle, and Psi, which confers an inherited state in yeast. Yeast Prions Show Unusual Inheritance • amyloid fibers – Insoluble fibrous protein polymers with a cross β-sheet structure, generated by prions or other dysfunctional protein aggregations (such as those fibrous protein polymers present in Alzheimer’s patients). • Conversion between the two forms of a given protein -can be influenced by action of “chaperones”. • The wild-type form has the recessive genetic state psi– and the mutant form has the dominant genetic state PSI+. Yeast Prions Show Unusual Inheritance • The Sup35 protein in its wild-type soluble form is a termination factor for translation. • Sup35 can also exist in an alternative form of oligomeric aggregates, in which it is not active in protein synthesis.
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